Heretofore, a flexible polyurethane foam having a low rebound resilience i.e. low resiliency, has been used for a shock absorber, a sound absorbent or a vibration absorber. Further, it is known that when it is used as a cushion material for chairs, mattress, etc., the body pressure distribution will be more uniform, whereby feeling of fatigue, pressure sores, etc. will be reduced. As an example, a low resilience polyurethane foam disclosed in Patent Document 1, is known.
The low resilience polyurethane foam is a low resilience polyurethane foam obtainable by reacting a composition of polyurethane foam materials comprising a polyol, a polyisocyanate, a catalyst and a blowing agent, and it has glass transition points within temperature ranges of from −70° C. to −20° C. and from 0° C. to 60° C., respectively. Further, it is disclosed that when the glass transition points are represented by peak values of tan δ obtainable by dynamic viscoelastic measurement at a frequency of 10 Hz, the peak value of tan δ within a temperature range of from −70° C. to −20° C. is at least 0.15, and the peak value of tan δ within a temperature range of from 0° C. to 60° C. is at least 0.3.
It is disclosed that the low resilience polyurethane foam has excellent low resiliency at room temperature, since it has a glass transition point within a temperature range of from 0° C. to 60° C., and there is not substantial increase in hardness at low temperatures, since it has a glass transition point within a temperature range of from −70° C. to −20° C.
However, a low resilience polyurethane foam having a glass transition point in the vicinity of room temperature has had a problem so-called temperature sensitivity such that as the working temperature departs from the glass transition point, the hardness tends to change, and the low resiliency tends to be unstable.
Further, in recent years, the level of durability required for a flexible polyurethane foam has become increasingly high. It is desired to further lower the resiliency and to develop a low resilience flexible polyurethane foam having a rebound resilience of at most 5%.
It is usually possible to lower the resiliency (rebound resilience) of a flexible polyurethane foam by incorporating a plasticizer to the flexible polyurethane foam. Accordingly, by adding a proper amount of a plasticizer, it is possible to obtain a flexible polyurethane foam having a desired low resiliency to some extent. However, the added plasticizer is likely to elute, for example, when the flexible polyurethane foam is washed, and, for example, it has been difficult to maintain the low resiliency of the flexible polyurethane foam after washing it repeatedly.
Further, a low resilience flexible polyurethane foam usually has low air permeability. Namely, it is known that the air permeability of a flexible polyurethane foam usually decreases as the resiliency decreases. In a case where a low resilience polyurethane foam is applied particularly to bedding, if the air permeability is low, humidity (mainly released from human body) tends to be hardly dissipated, thus leading to a so-called humid state. A low resilience polyurethane foam for bedding has been required to reduce such a humid state and to dissipate the heat and humidity. Further, when the usage state of bedding is taken into consideration, as a flexible polyurethane foam is to be used in a compressed state, it is required to exhibit substantially higher air permeability in a test for air permeability as measured usually in a non-compressed state. Further, in consideration of the fact that it is compressed in a humid state, the durability in a humid state is required. As an index for the durability in a humid state, the wet heat compression set may be mentioned.
As a method to solve the above problems and to improve the air permeability of a low resilience polyurethane foam, a method of employing a low molecular weight polyhydric alcohol as a raw material polyol has been proposed, as disclosed in Patent Documents 2 and 3. However, the low resilience polyurethane foam obtained by such a method has a problem with respect to the durability, and the restoration performance tends to gradually deteriorate. Further, in Patent Document 4, a low resilience polyurethane foam is obtained by using a polyether polyester polyol and a phosphorus-containing compound. However, the phosphorus-containing compound shows the same behavior as a plasticizer and is likely to elute from the flexible polyurethane foam, whereby it is expected to be difficult to maintain the performance after repeating the washing.
Further, Patent Document 5 discloses a method for producing a low resilience polyurethane foam having a good air permeability by using a monool in combination for the production. However, this method has a problem that the after-mentioned durability in a humidified state is poor. In Patent Documents 6 and 7, a method of employing a polyol composition containing a specific monool is proposed. However, in such a method, a low molecular weight polyether triol is used in a large amount, and the above-mentioned problem of temperature sensitivity has not yet been solved.
Patent Document 1: JP-A-11-286566
Patent Document 2: JP-A-2004-2594
Patent Document 3: JP-A-2004-43561
Patent Document 4: JP-A-9-151234
Patent Document 5: JP-A-2004-300352
Patent Document 6: JP-A-2003-522235
Patent Document 7: JP-A-2004-530767